What's required to make removal of a mounted USB stick safe?

[Julian H. Stacey]

Hi, Reference:
> From:		NGie Cooper <yaneurabeya at gmail.com>
> Date:		Wed, 6 May 2015 14:28:12 -0700

NGie Cooper wrote:
> On Wed, May 6, 2015 at 1:49 PM, Ryan Stone <rysto32 at gmail.com> wrote:
> > Currently FreeBSD stands a fair chance at panicking if a mounted USB drive
> > is removed while I/O is in flight.  Does anybody know what work is involved
> > to have the kernel safely recover from this case?  Losing data from the
> > drive is expected of course but there's no reason that the entire kernel
> > has to crash.
> >
> > A co-worker has been looking at this but I don't feel that we understand
> > the problem well enough to produce a real fix.  All that we've been doing
> > so far is papering over the explicit panics without having a full
> > understanding of what we're doing.
> 
> What version are you working on and how is the USB stick mounted (/, /mnt, etc)?

Not a new problem, it' been so over 30 years with Unix. 
Remove media without umount & game over.  
Some Solutions:
- Deep kernel work (dont hold your breath, see 30 above, & don't look at me :-)
- Cobble up some C to run from user space, not as root,
  so that your UFS is not mounted, but accessed by user level
  programs (much like mwrite & mread for accessing DOS media of old)
- man 8 amd : set a short timeout to auto unmount,
  it wont total solve your problem, but should lessen the frequency of panics.
- Mount the USB media on a spare laptop running as an NFS+ AMD server,
  then access the FS via NFS from your real client big PC.  When
  you pull the stick by accident forgetting its mounted, just the
  laptop crashes afte a bit, the AMD access on the other PC just
  hangs but doesnt crash.
- Encourage A SOC (google summer of code) student to look at it,
  probably wont come to a solution though, see 30 above.
- Toss money at the problem :-) If your company can afford some cash, either:
  - help fund FreeBSD Foundation & ask them to solve it, 
  - Or pay some consultant somewhere to look at it, Here's a globaly
    geographicaly indexed list http://berklix.com/consultants/
    maybe there's one near your company ?

Cheers,
Julian
--
Julian Stacey, BSD Linux Unix C Sys Eng Consultant Munich http://berklix.com
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